This invention relates generally to processes for the fluidized catalytic cracking (FCC) of heavy hydrocarbon streams such as vacuum gas oil and reduced crudes. This invention relates more specifically to a method for separately reacting a feed in a principally thermal cracking zone and another feed in a principally catalytic cracking zone.
The fluidized catalytic cracking of hydrocarbons is the main stay process for the production of gasoline and light hydrocarbon products from heavy hydrocarbon charge stocks such as vacuum gas oils or residual feeds. Large hydrocarbon molecules associated with the heavy hydrocarbon feed are cracked to break the large hydrocarbon chains thereby producing lighter hydrocarbons. These lighter hydrocarbons are recovered as product and can be used directly or further processed to raise the octane barrel yield relative to the heavy hydrocarbon feed.
The basic equipment or apparatus for the fluidized catalytic cracking of hydrocarbons has been in existence since the early 1940""s. The basic components of the FCC process include a reactor, a regenerator, and a catalyst stripper. The reactor includes a contact zone where the hydrocarbon feed is contacted with a particulate catalyst and a separation zone where product vapors from the cracking reaction are separated from the catalyst. Further product separation takes place in a catalyst stripper that receives catalyst from the separation zone and removes entrained hydrocarbons from the catalyst by counter-current contact with steam or another stripping medium.
The FCC process is carried out by contacting the starting materialxe2x80x94whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbonsxe2x80x94with a catalyst made up of a finely divided or particulate solid material. The catalyst is transported like a fluid by passing gas or vapor through it at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction. The cracking reaction deposits coke on the catalyst. Coke is comprised of hydrogen and carbon and can include other materials in trace quantities such as sulfur and metals that enter the process with the starting material. Coke interferes with the catalytic activity of the catalyst by blocking active sites on the catalyst surface where the cracking reactions take place. Catalyst is traditionally transferred from the stripper to a regenerator for purposes of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content relative to the catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated from the reaction zone to the regeneration zone and then again to the reaction zone. The fluidized catalyst, as well as providing a catalytic function, acts as a vehicle for the transfer of heat from zone to zone. Catalyst exiting the reaction zone is spoken of as being spent, i.e., partially deactivated by the deposition of coke upon the catalyst. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.
The FCC unit cracks gas oil or heavier feeds into a broad range of products. Cracked vapors from the FCC reactor enter-a separation zone, typically in the form of a main column, that provides a gas stream, a gasoline cut, cycle oil and heavy residual components. The gasoline cut includes both light and heavy gasoline components. A major component of the heavy gasoline fraction comprises heavy single ring aromatics.
It has long been desired to process more than one feedstock in an FCC unit. FCC processes have been proposed for cracking multiple feeds in a single riser. U.S. Pat. No. 4892643 specifically discloses the cracking of first a gas oil mixture followed by cracking of a naphtha boiling range stream in a single FCC riser. It is also known from U.S. Pat. No. 5389232 to use a heavy naphtha boiling range hydrocarbon as a quench in an FCC riser to control the riser temperature and the cracking of a gas oil feed.
Recent advances in FCC process arrangements have led to significant reductions in the amount of the coke laid down on the catalyst in the reaction zone. Improvements to the distribution of feed and the separation of products from catalyst have largely contributed to the reduction in coke production. While reduction in coke is desirable overall, it has the effect of limiting the operating temperature of the regeneration zone and the resulting temperature of the regenerated catalyst. Lower regenerated catalyst temperatures reduce the reaction temperature in the reactor riser. Lower reaction temperatures shift the cracking reaction away from thermal cracking and toward catalytic cracking. To maintain conversion it is often necessary to circulate more catalyst with the feed. Circulating more catalyst can be an imperfect solution to reduced conversion. First, the higher catalyst circulation rate may tend to further reduce coke lay down resulting in a downward temperature spiral for the regenerated catalyst as the temperature of the catalyst decreases with increased circulation and the circulation must continue to increase with decreasing catalyst temperature. In addition, in many existing units the catalyst circulation rate may be limited so that increasing the catalyst-to-feed ratio may come at the expense of limiting feed throughput.
Relatively lower regenerated catalyst temperature poses special problems for dual conversion zone arrangements. Circulation of the catalyst through an additional conversion zone will have an inherent cooling effect. Moreover, in the vast majority of cases the additional conversion zone will effect an endothermic reaction. Therefore, the additional conversion zone operates as a catalyst cooler that further removes heat from the process and continues the depression of regenerated catalyst temperatures. The problem becomes further exacerbated where the additional conversion would benefit from higher operating temperatures, such as in the case of thermal cracking, but high temperature catalyst is unavailable.
The available methods of increasing regenerated catalyst temperature are not commercially attractive. Reducing the hydrocarbon conversion and/or the recovery of hydrocarbons from the process will increase regenerator temperature, but at the expense of overall process efficiency. Various promoters and combustion material may be added to the regenerator to promote CO combustion or to combust additional fuel. Both of these alternatives add expense and complexity to the operation of the regenerator.
U.S. Pat. No. 2883332 describes the use of two separate bed type reaction zones in an FCC process and the charging of a recycle stock to one of the reaction zones and the recovery of the product streams from both of the reaction zones through a common recovery system.
U.S. Pat. No. 3161582 teaches the use of a riser reaction zone that converts a first feed and discharges the converted feed into a second bed type reaction zone that treats additional feed of a more refractory nature. All of the converted feeds are recovered from a common dilute phase collection zone in the reactor.
U.S. Pat. No. 2550290 discloses an FCC process that contacts an FCC charge oil in a first reaction vessel, separates the products from the first reaction vessel, and contacts the bottoms stream from the product separation in a separate second reaction vessel.
U.S. Pat. No. 2915457 describes the treatment of an FCC feed in a first riser type catalytic cracking vessel; separation of cracked hydrocarbons from the first vessel into a gasoline product, a heavy residual stream and a gas oil stream; hydrotreating of the gas oil stream; cracking of the hydrotreated gas oil in a second reaction vessel; and recycling of gas oil and heavier cracked components in the second reaction vessel.
U.S. Pat. No. 3607129 shows an apparatus for cracking a heavy FCC feedstock in a riser conversion zone, discharging the cracked product into an FCC reactor vessel, cracking hydrotreated or unhydrotreated light cycle oil in a fluidized catalyst bed in a lower portion of the reaction vessel and withdrawing the cracked products from the riser and the dense bed through a common conduit.
U.S. Pat. No. 4624771 discloses a riser cracking zone that uses fluidizing gas to pre-accelerate the catalyst, a first feed introduction point for injecting the starting material into the flowing catalyst stream, and a second downstream fluid injection point to add a quench medium to the flowing stream of starting material and catalyst.
U.S. Pat. No. 3776838 shows the cracking of a naphtha stream in a fluidized catalytic cracking process.
U.S. Pat. No. 5082983 teaches the introduction of a light reformate stream into an FCC riser.
U.S. Pat. No. 2915457 shows multiple-staged catalytic cracking of primary feed and a recycled, cracked product fraction in a separate catalytic cracking zone using spent catalyst from the primary cracking zone.
U.S. Pat. No. 4830728 shows the cracking of a primary FCC feed using one type of catalyst in a primary reaction zone and a cracking of a naphtha feed in a second riser reaction zone using a substantially segregated catalyst to independently recover separate primary and secondary feeds from the reaction zones.
U.S. Pat. No. 4990239 discloses an FCC process for improving the production of middle distillate fuels by recycling a hydrotreated and hydrocracked light cycle oil to the primary feed of the FCC reaction zone.
U.S. Pat. No. 5152883 shows a separate FCC reaction zone for the cracking of a primary FCC feed, the hydrogenation of a bottoms fraction from the cracked FCC product and the recracking of a further separated fraction from the hydrogenation zone effluent in a separate catalytic cracking zone.
U.S. Pat. No. 5401389 discloses a catalyst and method for upgrading light cycle oil to a low sulfur gasoline by hydrodesulfurization and hydrogenation for catalytic cracking of the light cycle oil fraction.
U.S. Pat. No. 5310477 discloses a riser reaction zone and a fixed bed reaction zone and a single reactor vessel for the catalytic cracking of a primary FCC feed and a heavy gasoline or light cycle oil feed that may undergo optional hydrotreating. The arrangement also shows the potential for separate recovery of the primary and secondary products in separate fractionation zones.
U.S. Pat. No. 5582711 discloses an FCC process that uses separate risers for the contacting of a primary feed and a hydrotreated product fraction recovered from the cracked product of the primary feed. The reactor arrangement delivers both products to a common fractionation column.
British reference UK 2216896 A teaches the charging of an FCC feed to an intermediate riser location and the charging of heavy slurry oil feed to a lower riser location.
It is an object of this invention to provide a fluidized catalyst process that operates dual conduit conversion zones and supplies regenerated catalyst at relatively high temperatures without the use of promoters or combustion materials.
It is a more specific object of this invention to operate a fluidized catalyst with a conduit conversion zone for principally thermal cracking and a conduit conversion zone for principally catalytic cracking that together produce catalyst with sufficient coke to provide regenerated catalyst at relatively high temperatures.
It is a further object of this invention to use an extended riser arrangement to provide one conduit section for converting a light feed such as a naphtha boiling range feedstream to olefinic products and to provide another conduit section for converting a traditional gas oil feedstream.
It is a further object of this invention to provide an FCC arrangement for conversion of naphtha in one riser conduit section, for intermediate recovery of a naphtha product, and for reuse of the catalyst that has contacted the naphtha feedstream in the catalytic cracking of a relatively heavier feed.
Accordingly, this invention is an FCC process for cracking multiple feeds. The process cracks one feed in one contacting conduit using a blend of catalyst that includes carbonized catalyst from a different contacting conduit as a portion of a catalyst blend to lay down enough coke on catalyst to provide regenerated catalyst with sufficient temperature to promote thermal cracking in one of the contacting conduits. In this manner, the invention provides a first contacting conduit section that can operate as a principally thermal cracking zone. A second contacting conduit utilizes the lightly to moderately coked catalyst from the first contacting conduit as a portion of its catalyst stream for principally catalytic cracking of another feedstream. The principally thermal cracking section benefits from the use of high temperature catalyst. The principally catalytic cracking section benefits from the use of carbonized catalyst that has had previous contact with feed in the thermal cracking conduit but retains ample activity to raise the available catalyst-to-oil ratio without increasing catalyst circulation through the regenerator.
The carbonized catalyst that is part of the catalyst mixture entering the second contacting conduit may circulate through the reaction side of the process along a variety of paths. Carbonized catalyst that, since its regeneration, has only had prior contact with the feed in the principally thermal cracking zone is referred to as xe2x80x9ccontacted catalystxe2x80x9d. Carbonized catalyst refers more generally to catalyst that has been coked by a single passage through the principally thermal cracking zone and catalyst that has passed through either or both of the principally thermal cracking zone and the principally catalytic cracking zone. Carbonized catalyst is usually referred to as xe2x80x9cspent catalystxe2x80x9d. However, the carbonized catalyst retains activity and therefore the term xe2x80x9cspent catalystxe2x80x9dxe2x80x94while generally acceptedxe2x80x94is misdescriptive. It is the intention of this invention to more fully utilize this remaining activity by returning what is herein termed xe2x80x9ccarbonizedxe2x80x9d and xe2x80x9ccontactedxe2x80x9d catalyst back to a reaction zone without any regeneration. Carbonized catalyst will eventually undergo stripping after contact with feed in one or more of the contacting zones. Catalyst returning to the contacting conduit from the stripping zone is referred to as recycle catalyst.
The contacted catalyst retains high activity while providing additional catalyst for highly desired passivated contact of the heavy feed at high catalyst-to-oil ratios. Furthermore, the large catalyst-to-oil ratio provided by the contacted catalyst and the recycle catalyst provides a moderated temperature that remains stable due to the high volume of catalyst present in the primarily catalytic contacting zone. The recycle of the contacted catalyst from the downstream portion of the upstream contacting zone has the additional benefit of lowering the overall catalyst temperature of the thermally cracked feed and catalyst mixture as it exits the upstream contacting zone. The mixing of the contacted catalyst with the recycle catalyst provides a quenching effect on the reaction of the lighter feed component as it exits from relatively higher temperature operating conditions of the upstream contacting conduit.
The contacting conduit that contains the principally thermal cracking reaction, hereinafter referred to as the thermal conduit, passes the catalyst that it discharges into a blending vessel. The blending vessel may directly receive catalyst discharged from the thermal conduit or may receive a mixture of contacted and recycle catalyst from both the thermal conduit and the catalytic conduit, i.e. the contacting conduit that contains the principally catalytic reaction. In either case the blending vessel provides thorough mixing of the contacted catalyst stream from the thermal conduit and recycle catalyst that passes from the outlet of the catalytic conduit. Locating the blending vessel at the downstream end of the thermal conduit will position the blending vessel to receive a direct discharge of catalyst from the thermal conduit. The catalytic conduit may be located immediately upstream of the blending vessel so that the blending vessel separates an upstream thermal conduit and a downstream catalytic conduit. The blending vessel may be arranged to provide independent withdrawal of the cracked products from the thermal conduit. Such an arrangement at least partially segregates vapors from the thermal conduit from the entering feed of the catalytic conduit. Vapors from the thermal conduit that pass into the catalytic conduit may serve as a lift medium for carrying the blended mixture of catalyst through the catalytic conduit.
Whether arranged for separate recovery of thermally cracked and catalytically cracked streams or combined recovery of catalytic and thermally cracked streams the effluent from both conduits will pass through a catalyst separation zone. The catalyst separation zone may comprise any type of catalyst separation such as ballistic or centrifugal separation. The separation will preferably offer a high degree of containment to control residence time and prevent overcracking.
After catalyst separation the fluid from the contacting conduits will pass to a fluid separation zone. The fluid separation section may have separate vessels for separating independently recovered thermally cracked lighter product and an independently recovered catalytically cracked product. Alternately, the separation zone may recover a full range of products from a combined fluid that contains both the effluent of the thermal and downstream catalytic conduits. The separation zone may also provide all or a portion of the feed to the thermal conduits as well as recycle materials for return to the catalytic conduit.
The upstream section of the contacting conduit may crack a variety of different feeds. In most cases the feed to the thermal conduit will have a lower average boiling point than feed to the catalytic conduit. The catalytic conduit ordinarily receives a traditional gas oil feed. Feeds for the thermal conduit will usually comprise light cycle oils and various middle distillate boiling range cuts having a boiling range of from 400-700xc2x0 F. or naphthas boiling in a range of from 80-450xc2x0 F. Naphthas are usually preferred feeds and this invention may produce valuable light products from a variety of feeds to the thermal conduit including a mid-boiling range naphtha (250-360xc2x0 F.), a high boiling range naphtha (350-430xc2x0 F.), and a full boiling range naphtha (100-430xc2x0 F.).
The conditions within the thermal conduit will typically provide high catalyst-to-oil ratios that maximize the temperature available from the regeneration zone for the principally thermal cracking of the feed. Regenerated catalyst will typically enter the thermal conduit in a sufficient amount to produce a catalyst-to-oil ratio in a range of from 12/1 to 150/1 and preferably in a range of from 20/1 to 50/1. Regenerated catalyst entering the upstream portion of the contacting conduit will usually have a temperature of at least 1330xc2x0 F. and, once blended with the lower boiling range feed, will produce an average temperature of from 1225-1350xc2x0 F. in the high severity contacting conduit. Contact between the feed and catalyst in the upstream contacting conduit will usually be in a range of from 0.5 to 5 seconds and, preferably, will be in a range of from 2 to 3 seconds.
Repeated contact and blending of the contacting catalyst with recycle catalyst will ordinarily increase the average coke content of the spent catalyst that passes to the regenerator. After recycle and return, spent catalyst entering the regenerator will have from 0.2 to 0.4 wt-% more coke on catalyst than is currently obtained from a modern FCC operation processing a feedstock with average coking tendencies. Preferably, the spent catalyst that passes from the reaction of the process to the regenerator will have a coke content of at least 0.8 wt-% and, more preferably, will have a coke content of at least 0.9 wt-%.
Accordingly, in one embodiment, this invention is a fluidized cracking process for the principally thermal cracking of a secondary feed and for the principally catalytic cracking of a primary feed in an arrangement of separate reaction conduits. The secondary feed is typically a light feedstock, preferably a naphtha boiling range, and the primary feed is typically a relatively heavier feedstock. The process comprises passing the secondary feed and regenerated catalyst particles to an upstream portion of a thermal contacting conduit and transporting the regenerated catalyst and secondary feedstock through the thermal contacting conduit to convert the feed to a thermal fluid while producing a first quantity of contacted catalyst particles by the deposition of coke on the regenerated catalyst particles. The thermal contacting conduit discharges the contacted catalyst particles and the thermal fluid from a discharge end. The contacted catalyst particles pass to a blending vessel for blending with a carbonized catalyst which produces a blended catalyst stream. The blended catalyst stream passes from the blending vessel into a catalytic contacting conduit that contacts the blended catalyst mixture in the catalytic contacting conduit with the primary feed to produce a mixture of catalyst and catalytic fluid. A primary catalyst separation zone separates catalyst from the mixture of catalyst and catalytic fluid for the recovery of a primary effluent stream from the primary catalyst separation zone. The process recovers spent catalyst for regeneration in a regeneration zone and the process passes the primary effluentxe2x80x94and optionally a separately recovered portion of the thermal fluidxe2x80x94to a fluid separation zone to recover an olefin product stream comprising ethylene and/or propylene and a primary product stream.
In a more limited embodiment, this invention is a process for the fluidized catalytic cracking (FCC) of a light feedstock, usually naphtha, and a relatively heavier feedstock in a series flow conduit arrangement. The process passes the light feedstock and regenerated catalyst particles to an upstream portion of a secondary contacting conduit and transports the regenerated catalyst and light feedstock through the secondary contacting conduit to convert the light feedstock to a principally thermal cracked fluid. Deposition of coke on the regenerated catalyst particles produces contacted catalyst particles. The contacted catalyst particles and the principally thermal cracked fluid are discharged from a discharge end of the secondary contacting conduit into a blending vessel and blended with carbonized catalyst to produce a blended catalyst stream. The blended catalyst stream passes from the blending vessel into a primary contacting conduit that contacts the blended catalyst mixture with a heavy feed having a higher average boiling point than the light feed to produce a mixture of carbonized catalyst and a principally catalytically cracked effluent. Separating catalyst from the mixture in a primary catalyst separation zone provides recovery of a primary effluent stream from the primary catalyst separation zone. The primary productxe2x80x94and optionally a separately recovered portion of the principally thermally cracked fluidxe2x80x94passes to a fluid separation zone for recovery of a light product stream comprising propylene and ethylene and a heavy product stream. In a more narrow form of this embodiment, the fluid separation zone includes at least two fractionation sections and the principally thermally cracked fluid and the primary effluent pass to separate fractionation sections. Where the lighter feed comprises naphtha, cracking of the lighter stream produces propylene and ethylene in a combined yield of 15-25 wt-% of the naphtha feed or 10-25 wt-% of the principally thermally cracked fluid.
In another aspect of this invention, a naphtha feedstock is the first feed to pass through a series flow riser arrangement. The naphtha stream and regenerated catalyst particles pass to a thermal cracking riser and travel up the lower riser portion to convert the naphtha feedstock to a cracked naphtha effluent. A quantity of contacted catalyst particles and the cracked naphtha effluent enter a blending vessel that blends a quantity of carbonized catalyst with the quantity of contacted catalyst to produce the blended catalyst stream. The blended catalyst stream passes from the blending vessel into a catalytic cracking riser where the blended catalyst mixture contacts a heavy feed having an average boiling point in a range of from 600-1150xc2x0 F. to produce carbonized catalyst and a heavy cracked effluent. Separation of catalyst from the mixture in the primary catalyst separation zone provides a primary effluent stream that passes to a fluid separation zone.
In another embodiment, this invention is an apparatus for the fluidized cracking of a light feedstock and a heavy feedstock. The apparatus arranges a first contacting conduit section defining a discharge outlet at its downstream end with a first feed conduit for delivering a first feed to the first contacting conduit section. An intermediate portion of the conduit serves as a blending section that directly communicates with the discharge outlet of the first contacting conduit and connects to a catalyst inlet at an upstream end of a second contacting conduit section to provide direct communication with the blending section. A second feed conduit charges a second feedstream to the blending section or the second contacting conduit section. A primary catalyst separator receives a mixture of catalyst and vapors from the discharge end of the second contacting conduit section. At least one fluid separator separates cracked vapors from the second contacting conduit section into a light product stream and a heavy product stream. In addition, an intermediate recovery line may communicate with the blending section to recover a separate light product such as a cracked naphtha product. The first and second contacting conduit sections will usually comprise risers for the upward transport of catalyst and fluids. Ordinarily the blending section has a larger diameter than the first and second contacting conduit sections.
Other objects, embodiments, and details of this invention are set forth in the following detailed description of the invention.